The power output of an optical parametric oscillator (OPO) is predicted to be path length dependent. Experimental data for a particular OPO configuration show that path lengths must be maintained to within +/−50 micrometers of the peak position to maintain a power level of 95% of the peak power. To stay within 95% of the relatively stable portion of the power curve, slightly below peak power, path length changes of approximately +/−150 micrometers can be tolerated. The power output of another OPO configuration is even more path length dependent requiring path length changes of only +/−10 micrometers to maintain power levels to within 95% of the peak power. To stay within 95% of the relatively stable portion of the power curve, path length changes of approximately only +/−35 micrometers can be tolerated.
The path length of an OPO can be very dependent on thermal conditions, so athermalization is necessary. In one OPO, the unused pump energy of the IR pump source, as well as the idler energy is absorbed in a beam dump, contained within the OPO cavity. The beam dump is cooled using a closed loop chiller system, in which the chiller water first passes through the bulk of the OPO cavity (in channels running through the base plate), and is then in direct contact with the beam dump. After contacting the beam dump, the coolant returns to the chiller. Because of thermal loading, the chiller temperature is found to increase by approximately 2° C. over an 8-hour period. The coolant never reaches an equilibrium temperature, although the temperature rise slows with time. The output power reaches 95% of the final value within 1.5 hours of start up.
Thermal modeling of the OPO indicates that path lengths are strongly affected by thermal loading and the cooling configurations. Coolant temperature has a strong influence on the optical path length in the OPO, with a 2° C. rise in coolant temperature resulting in an approximately 75 micron path length change. Coolant temperature is determined by the chiller set point and by thermal loading due to the OPO internal heat sink. Ambient air temperature has only a very slight effect on the path length, with temperature rises of approximately 5° C. resulting in only a 2 micrometer increase in path length.
The path length of an OPO can be very dependent on thermal conditions. Thermal modeling of the OPO indicates that path lengths are strongly affected by thermal loading and the cooling configurations. Coolant temperature has a strong influence on the optical path length in the OPO, with a 2° C. rise in coolant temperature resulting in an approximately 75 micron path length change. The strong dependence of path length on coolant temperature and thermal loading result in a system that is not robust in terms of output power. In addition, long warm up times are required so that coolant temperature comes close to equilibrium. It will be appreciated that it would be highly desirable to have a method for controlling temperature to achieve a optical parametric oscillator that is robust in terms of power output.